1. Field of the Invention
The present invention relates to a pump and a pump unit comprising a pump and a driving mechanism and, more particularly, to a pump comprising a pump body and moving members slidably fitted in the pump body to form a chamber in the pump body, and a pump unit comprising such a pump and a driving mechanism.
2. Description of the Related Art
A suction pump of the type to which the present invention is related for suctioning ink is disclosed in European Laid-open Patent Publication Nos. 589541 and 375407. This suction pump is intended for use in an ink-jet recording apparatus to suction the ink remaining in the discharge ports of an ink-jet head and the ink remaining in an ink chamber. As shown in FIGS. 10 and 11, this suction pump comprises a cylindrical pump body 300, and a piston 302, i.e., a moving member, slidably fitted in the pump body 300 in a liquid-tight fashion and provided with a discharge port 310. The pump body 300 and the piston 302 define a suction chamber 304. When the suction pump operates for suction, a driving shaft 308 having a first head 312 and a second head 318 is moved to the right, as viewed in FIG. 10, by a driving mechanism, not shown. The discharge port 310 of the piston 302 is closed by the first head 312 of the driving shaft 308, and at the same time, the piston 302 is moved to the right with the first head 312. The volume of the suction chamber 304 increases and the pressure in the suction chamber 304 decreases as the piston 302 is moved to the right. Consequently, the ink is suctioned through a suction port 316 formed in the pump body 300 into the suction chamber 304 when the suction port 316 is opened. Then, as shown in FIG. 11, the driving shaft 308 is moved to the left. The second head 318 of the driving shaft 308 comes into contact with the end face 320 of the piston 302, and the first head 312 of the driving shaft 308 separates from the other end face 322 to open the discharge port 310 into the suction chamber 304. The piston 302 is moved to the left with the second head 318, and the volume of the suction chamber 304 decreases accordingly, so that the ink suctioned into the suction chamber 304 is discharged through the discharge port 310.
In this suction pump, the suction port 316 is opened and closed with the piston 302, and the discharge port 310 is opened and closed with the first head 312 formed integrally with the driving shaft 308. Thus, the suction port 316 and the discharge port 310 can be more reliably opened and closed with the piston 302 and the first head 312, respectively, than by a suction valve and a discharge valve, which are controlled by the ink. Since the suction port 316 and the discharge port 310 are opened and closed by the piston 302 that varies the volume of the suction chamber 304 and the first head 312, respectively, this suction pump does not need any valve driving mechanism for driving a suction valve and a discharge valve and has a simple construction.
However, this suction pump has a problem that arises unavoidably due to the use of the piston 302 for opening and closing the suction port 316 and the use of the first head 312 formed integrally with the driving shaft 308 for opening and closing the discharge port 310. For example, part of the ink suctioned into the suction chamber 304 flows backward unavoidably through the suction port 316. As mentioned above, when discharging the ink, the driving shaft 308 is moved to the left from the position shown in FIG. 10, and the first head 312 separates from the end face 322 of the piston 302 to open the discharge port 310. However, both the discharge port 310 and the suction port 316 are open at the moment when the discharge port 310 is opened, and the driving shaft 308 is moved to the left with the suction port 316 open to reduce the volume of the suction chamber 304. Consequently, part of the ink suctioned in the suction chamber 304 flows backward through the suction port 316 until the suction port 316 is closed by the piston 302. Furthermore, the volume of the suction chamber 304 of the suction pump at the completion of the discharge operation is not satisfactorily small, because the volume of the suction chamber 304 at the completion of the discharge operation cannot be reduced to a volume smaller than a volume corresponding to the distance d between the first head 312 and the end face 322 of the piston 302 in a state where the discharge port 310 is open (the valve lift), i.e., the product of the sectional area of the suction chamber 304 and the distance d. As mentioned above, the discharge port 310 is closed when the first head 312 of the driving shaft 308 comes into contact with the end face 322 of the piston 302; and the discharge port 310 is opened when the first head 312 is separated from the end face 322 of the piston 302, and the piston 302 is moved to the left with the second head 318 of the driving shaft 308 that pushes the piston 302 at the end face 320. Therefore, the length between the first head 312 and the second head 318 of the driving shaft 308 must be greater than the length of the piston 302, i.e., the distance between the end faces 320 and 322, by the valve lift amount d. Therefore, when the driving shaft 308 is moved to the left end position, a space of a width equal to the valve lift d remains between the end face 322 of the piston 302 and the first head 312, which is regarded as the bottom wall of the pump body 300, and a comparatively large quantity of the ink is left in the suction chamber. The ink left in the suction chamber will be called the residual ink.
Naturally, this suction pump can be used for supplying liquid other than the ink or gas, which will be referred to as "fluid," by pressure as well as for suctioning fluid, and the aforesaid problem arises therein when the suction pump is used for supplying a fluid by pressure.